3GPP-LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution, hereinafter referred to as “LTE”) adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme and adopts SC-FDMA (Single Carrier Frequency Division Multiple Access) as an uplink communication scheme (e.g. see non-patent literatures 1, 2 and 3).
According to LTE, a radio communication base station apparatus (hereinafter abbreviated as “base station”) performs communication by allocating resource blocks (RBs) in a system band to a radio communication terminal apparatus (hereinafter abbreviated as “terminal”) per time unit called “subframe.” Furthermore, the base station transmits control information for notifying results of resource allocation of downlink data and uplink data to the terminal. This control information is transmitted to the terminal using a downlink control channel such as PDCCH (Physical Downlink Control Channel). Here, each PDCCH occupies a resource made up of one or a plurality of continuous CCEs (Control Channel Elements). LTE supports a frequency band having a width of maximum 20 MHz as a system bandwidth.
Furthermore, the base station simultaneously transmits a plurality of PDCCHs to allocate a plurality of terminals to one subframe. In this case, the base station includes CRC bits masked (or scrambled) with destination terminal IDs to identify the respective PDCCH destination terminals in the PDCCHs and transmits the PDCCHs. The terminal demasks (or descrambles) the CRC bits in a plurality of PDCCHs which may be directed to the terminal with the terminal ID of the terminal and thereby blind-decodes the PDCCHs and detects a PDCCH directed to the terminal.
Furthermore, studies are being carried out on a method of limiting CCEs to be subjected to blind decoding for each terminal for the purpose of reducing the number of times blind decoding is performed at the terminal. This method limits a CCE area to be subjected to blind decoding (hereinafter referred to as “search space”) for each terminal. Thus, each terminal needs to perform blind decoding only on CCEs in a search space allocated to the terminal, and can thereby reduce the number of times blind decoding is performed. Here, the search space of each terminal is set using the terminal ID of each terminal and a hash function which is a function for performing randomization.
Furthermore, as for downlink data from the base station to the terminal, the terminal feeds back an ACK/NACK signal indicating an error detection result of the downlink data to the base station. The ACK/NACK signal is transmitted to the base station using an uplink control channel such as PUCCH (Physical Uplink Control Channel). Here, studies are being carried out on associating CCEs with a PUCCH to eliminate the necessity of signaling for notifying the PUCCH used to transmit an ACK/NACK signal from the base station to each terminal and thereby efficiently use downlink communication resources. Each terminal can decide a PUCCH used to transmit an ACK/NACK signal from the terminal from the CCE to which control information directed to the terminal is mapped. The ACK/NACK signal is a 1-bit signal indicating ACK (no error) or NACK (error present), and is BPSK-modulated and transmitted.
Furthermore, standardization of 3GPP LTE-Advanced (hereinafter referred to as “LTE-A”) has been started which realizes further speed enhancement of communication compared to LTE. LTE-A is expected to introduce a base station and a terminal (hereinafter referred to as “LTE+ terminal”) communicable at a wideband frequency of 40 MHz or above to realize a downlink transmission rate of maximum 1 Gbps or above and an uplink transmission rate of maximum 500 Mbps or above. Furthermore, the LTE-A system is required to accommodate not only an LTE+ terminal but also terminals compatible with the LTE system.
LTE-A proposes a band aggregation scheme whereby communication is performed by aggregating a plurality of frequency bands to realize communication in a wideband of 40 MHz or above (e.g. see non-patent literature 1). For example, a frequency band having a bandwidth of 20 MHz is assumed to be a basic unit (hereinafter referred to as “component band.” Therefore, LTE-A realizes a system bandwidth of 40 MHz by aggregating two component bands.
Furthermore, according to LTE-A, the base station may notify resource allocation information of each component band to the terminal using a downlink component band of each component band (e.g. non-patent literature 4). For example, a terminal carrying out wideband transmission of 40 MHz (terminal using two component bands) obtains resource allocation information of two component bands by receiving a PDCCH arranged in the downlink component band of each component band.
Furthermore, according to LTE-A, the amounts of data transmission on an uplink and downlink are assumed to be independent of each other. For example, there may be a case where wideband transmission (communication band of 40 MHz) is performed on a downlink and narrow band transmission (communication band of 20 MHz) is performed on an uplink. In this case, the terminal uses two downlink component bands on the downlink and uses only one uplink component band on the uplink. That is, asymmetric component bands are used for the uplink and downlink (e.g. see non-patent literature 5). In this case, both ACK/NACK signals corresponding to downlink data transmitted with the two downlink component bands are transmitted to the base station using ACK/NACK resources arranged on a PUCCH of one uplink component band.